1. Field of the Invention
The present invention relates to an optical film, an antireflection film, and a polarizing plate and a display device using the same.
2. Description of the Related Art
With recent increase in the screen size of a liquid crystal display device (LCD), a liquid crystal display device having disposed thereon an optical film such as antireflection film and light-diffusing sheet is increasing. For example, in various image display devices such as liquid crystal display device (LCD), plasma display panel (PDP), electroluminescent display (ELD) and cathode ray tube display device (CRT), the antireflection film is disposed on the display surface so as to prevent the reduction in contrast due to reflection of outside light or projection of an image. Also, the light-diffusing sheet is used on the backlight side of a liquid crystal display device.
The optical film is usually produced by stacking a light-diffusing (hardcoat) layer, a high refractive index layer, a low refractive index layer and the like on a transparent support.
The optical film is used on the outermost surface of a display and therefore, required to satisfy various film strengths, for example, scratch resistance against fine scratching and film hardness high enough to endure the pressure when written with a writing tool.
In order to meet these requirements, a method of stacking a hard layer on the surface or a method of incorporating an organosilane compound or increasing the thickness of the layer stacked has been performed.
On the other hand, the support of the surface film is required to be thinned for realizing a thin display surface.
When such a method of imparting film strength or thinning the support is performed, curling is increased and this brings about a problem that the surface film is hard to deal with in the production or processing, cracking (brittleness) is generated in the film during handling, or a separation phenomenon occurs after the lamination. In order to solve such a problem, there are created inventions using a modified acrylate-based compound (see, JP-A-2005-181996 (the terms “JP-A” as used herein means an “unexamined published Japanese patent application”)), a urethane acrylate-based compound (see, JP-A-2005-288787) or a mixture of acrylate-based compounds differing in the property (see, JP-A-2005-103979 and JP-A-2003-335983).
In these inventions, the curling-improving effect and improvement of brittleness and the like may be obtained, but the film surface hardness is decreased. It is difficult to simultaneously satisfy the surface hardness as well as those properties, and technical development of a material having sufficiently high film strength for the optical film is demanded.